Biot介质中井外声源激发井孔声场的理论研究与数值模拟

1、Biot介质中井外声源激发井孔声场的理论研究与数值模拟        【中文摘要】从理论上研究不同类型声源产生的地震波与井孔的相互作用,是典型的地震波散射题目,它对解释垂直地震剖面法和井间地震的丈量资料有直接的意义。本文依照Biot理论,针对孔隙地层中井外爆炸点源、井外点力源及两口井的井内居中爆炸点源三种地震模型进行了理论研究和数值模拟。考虑接收器所在的井均为*眼井情况,对三种模型的井孔内外的声场进行了具体的理论推导并给出了计算公式,并针对不同源距、不同声源主频和不同地层参数对井内声场进行了全波模拟和特性分析,其中

2、重点研究了井外点力源模型,首先考察了井外的直达波,发现这时的直达波有快纵波、慢纵波和SV波,但起主导作用的是快纵波和SV波;然后在不同条件下考察井内的全波特征,并对孔隙度、渗透率及方位角等影响全波波形的因素进行了考察和比较,进行了分析和描述;最后进行了频率波数分析,从而对井内的各极波场的相速度大小和频散现象有更直观的了解。本文在研究两口井的井间地震模型时,只考虑了井间一次散射的情况,通过波场模拟发现井间地震模型的声波场特征与井外点力源模型的声波场特征极其相似。本文针对不同的地震模型进行理论研究和模拟,有利于把握井间地震中波场的传播规律和特性,对实际波场的识别与分离也具有一定的指导意义。'

3、;);【Abstract】 Acoustic logging is a variety of specific acoustic technology logging collectively. The theoretical foundation is the elastic Waves excitation and propagation in the columnar layered medium. Theoretically studies the rule of acoustic excitation, propagation and reception in the borehol

4、e. The acoustic logging theorys main content researches the interaction of the elastic wave and the borehole entrance from different angle, so we should not only consider the different type of acoustic source, but also consider the type of stratum medium and the borehole systems geometry structure a

5、nd so on in theoretical research. This work is also the main subject in the theoretical studies of acoustic logging.For many years , acoustic logging get a rapid development. Conventional acoustic logging put acoustic sources and detectors in the same borehole, but acoustic logging is not very good

6、on stratum far from the hole. Although seismic on the ground can detect large range, but it has not high precision on stratum. Cross well is one of methods that can remedy the lack of acoustic logging and seismic. It put up a bridge between acoustic logging and seismic. Measuring the stratum across

7、the borehole and reservoir specificity can remedy the lack of cross detectability in the borehole. It also can settle many questions such as the unbalance of gathering the information type and the information abundance between the borehole and the cross well.So we should establish physics model base

8、 on the fact circs on cross well seismic. Resolving and computing acoustic field radiation, propagation and effect between boreholes from strict wave-equation and further studies on the rule of sound wave in cross well seismic are necessary. We should research the dielectric feature out the borehole

9、 and the effect stratum inte*ce to the full wave field and the partial wave field in the borehole. This work can make the mode perfect and help us comprehend the actual measuring method and the results. My subject launches based on such goal.In this * I discuss apart three models in the Biot solid t

10、hese excited by an external explosive point source of cross well, the model of external single point force source, the model of explosive point source located at axis in two wells through theoretical analysis and numerical simulation. Actually all these models belong to the model of cross well seism

11、ic.In the model that excited by an external explosive point source of cross well, the theoretical expressions were solved in and out borehole about all of frequency. The direct field only have longitudinal wave. According to the theoretical expressions, I found that the acoustic field in borehole ex

12、cited by an external explosive point source is quite to the summation of acoustic field excited by infinite orders multipole sources located at the axis of borehole. This article puts up the numerical simulation and frequency wave-number analysis of full wave to acoustic sound field in the borehole

13、which selects different center frequency, different radial spacing and different stratum. The stimulation shows that we must take into account more order of multipole term with increasing of center frequency, different stratum, receive distance offset the axis of borehole and degree of eccentricity.

14、 At the same time, Results of numerical analysis showed that the amplitude of Stoneley excited by the borehole will increase when reduce the center frequency in the same distance and stratum. The rule is uniform as acoustic logging. The amplitude of Stoneley wave excited in the borehole and other wa

15、ves will be reduced even disappeared with the increasing of the distance. It will only left the longitudinal wave because the effect between the spherical wave excited by an external explosive point source of cross well and the borehole become lower with the increasing of the distance. The amplitude

16、 of Stoneley wave is larger in hard stratum than in soft stratum in the same distance and center frequency.The model of external explosive point source of cross well can only measure the speed of longitudinal wave. It cant reflect all the stratum information accurately. This * has simulated the mode

17、l of external single point force source about this lack. The direct field is not only has longitudinal wave but also has SV wave in this model of external single point force source. It can receive longitudinal wave, transverse wave and all kinds of mode waves. We solve the theoretical expressions in

18、 and out borehole about all of frequency. The acoustic field in borehole excited by an external explosive point source is quite to the summation of acoustic field excited by infinite orders multipole sources located at the axis of borehole. Results of numerical analysis showed that the effect by dif

19、ferent center frequency, different stratum and receive distance offset the axis of borehole in model of external single point force source is very resemble to the model of external explosive point source. It can receive longitudinal wave, transverse wave and all kinds of mode waves. We can acquire d

20、etails about configuration. We can receive critical refractive longitudinal wave and receive critical refractive longitudinal wave in soft and hard stratum. The amplitude of transition critical longitudinal wave main associated with center frequency of sound source, stratum parameter, incident trans

21、versal waves strength. At the same time, I also considered the influence to full wave about some pore parameter and the position angle. The results show that porosity and the rate of permeation widely impact Stoneley wave and dipole flexural wave. In numerical simulation of frequency-wave number, it

22、 obviously shows that the comparation of phase velocity about the model waves of unipolar field, dipole field and four-pole field and dispersive situation. Through numerical simulation and analysis, we should comprehend the feature of seismic wave field in the pore stratum borehole. It is advantageo

23、us to realize and separate the seismic field in the borehole and receive the seismic field spread along well direction. That can help us obtain the details of reservoir and the stratum structure.At last, we studied the model of explosive point source located at axis in two wells in theory. Its direc

24、t field has longitudinal wave and SV wave. At the same time, Critical refractive longitudinal wave , Critical refractive longitudinal wave and all kinds of mode waves are received in the other well. Acoustic field has become comparatively infirmness once transmission and scattering. So we only calculate one time scattering. We solve the theoretical ex